Image-reading apparatuses that use a line sensor group composed of short line sensors arranged in a staggered pattern in a lengthwise direction to read a paper document is in use. For such kind of image-reading apparatus, generally the lengthwise direction of the line sensor (array direction) is referred to as the main scanning direction whereas the direction that is perpendicular to the lengthwise direction of the line sensor is referred to as the sub-scanning direction. The detection ranges of the line sensors, which are disposed in the main scanning direction of the line sensor group, partially overlap with each other as viewed from the sub scanning direction.
The paper document is relatively scanned in the sub-scanning direction by the line sensor group, enabling each line sensor to read a portion of the paper document. The images read by the line sensors are combined together at the overlapping portions and finally are synthesized into a single page of image data.
A method for detecting a position for combining images relating to the sub-scanning direction based on a degree of correlation in image-matching between the images captured by the line sensors is disclosed as a method for combining images read by the line sensors (for example: refer to Patent Literature 1).